Ciudad De Las Almas Perdidas by Cassandra Clare

After a devastating betrayal leaves one of their own under the control of a dangerous enemy, a group of Shadowhunters and their allies race to rescue him before he can be used to unleash a greater threat; amid frantic battles, betrayals and moral compromises they are forced to confront painful truths about loyalty, love and the costs of their war, while friendships are tested and unexpected alliances form in the struggle to stop a plan that could upend the world of Shadowhunters and Downworlders alike.